1. Field of the Invention
This invention relates to methods of making thin metal foil lead frames of the type which are used as electrical connecting devices.
2. Description of the Prior Art
The development of large scale integration (LSI) circuit devices or wafer devices mounted on a base provide an important advance in the electronic art because of the ability to provide varied and complicated circuits containing a plurality of elements in relatively small units. Problems exist of packaging these devices, particularly with respect to providing conductive paths to the numerous terminal points and to support them properly. One approach to this problem is to provide a device known as a lead frame, which is a member supporting a plurality of conductive leads in a prearranged pattern to which the integrated circuit device could be connected.
While the fabrication of lead frames for integrated circuit devices appears relatively simple in principle, the processing operations present many practical difficulties in regard to the selection of proper materials and the fabrications thereof into suitable form. Further, due to the limited space requirements, the circuitry is usually very dense. This imposes serious constraints on width and thickness of the conductive elements of the lead frame which are designed to make contact with contact areas on the LSI devices.
In the prior art, U.S. Pat. No. 3,778,899 teaches a process of fabricating a circuit pattern on a base member by electrodeposition and subsequently transferring the circuit pattern to a flexible dielectric substrate. U.S. Pat. No. 3,602,635 teaches the use of a photoresist layer in a microcircuit device to shield the sharp edges of metal bodies thereunder. Also, U.S. Pat. No. 3,537,175 relates a layer of relatively soft conductive material deposited longitudinally on a narrow strip of flexible metal material. The strip is stamped to form a plurality of integrally connected lead frames with narrow lead portions adapted for bonding to substrate supporting tips. No patents are known wherein a thin metal foil is adhered to a stiffener panel through the medium of a photoresist layer to enable processing steps by which the metal foil is transformed into an appropriate lead frame configuration.